Method for production of metallic elements of high purity such as chromes

ABSTRACT

The invention relates to a method of producing granules made of high purity metal or metal alloy, in particular based on chromium, the method being characterized in that it comprises the steps consisting in: preparing a metal or a metal alloy having non-metallic inclusions essentially comprising oxides of the base metal; pelletizing the metal or the alloy with a reducing agent in order to form the pellets or tablets; treating the pellets or tablets in a vacuum in order to enable the reducing agent to react on the inclusions without substantial sublimation of the metal or the metals of the alloy; and eliminating a surface layer from the pellets or tablets. The invention is applicable to manufacturing mechanical parts out of superalloys.

The invention relates to a method of producing metals or metal alloys ofhigh purity, and in particular metallic chromium.

Industry is requiring ever increasing quantities of metals and metalalloys of high purity for fabricating metal parts.

In document EP-0 102 892, the Applicant has described a method enablingvarious metals and alloys, in particular, chromium to be produced withhigh purity. That method comprises the steps consisting in:

-   -   a) preparing a metal or metal alloy in which the non-metallic        inclusions are essentially oxides of the base metal;    -   b) grinding the resulting metal or metal alloy and mixing the        ground metal or metal alloy with a pelletizing agent and a        reducing agent in order to form pellets; and    -   c) subjecting the pellets to reducing treatment in a vacuum        under conditions of pressure and temperature that are adjusted        so that the reducing agent reacts on the non-metallic        inclusions, and so that there is no substantial sublimation of        the treated metal or metals of the alloy.

By way of example, that method enables a product containing no more than300 parts per million (ppm) to 400 ppm of total oxygen to be obtainedwithout difficulty, the oxygen being in the form of about 200 ppm to 300ppm alumina containing 100 ppm to 150 ppm oxygen and about 500 ppm atmost of non-reduced chromium oxide containing about 150 ppm of oxygen.Consequently, that chromium is of high purity and is suitable for use inpreparing superalloys that are usable in particular in the manufactureof noble parts for aviation turbo-engines.

That method has given and continues to give full satisfaction.

Nevertheless, there now exists a demand for metals and metal alloys ofeven greater purity.

An object of the invention is thus to provide metals and metal alloys ofimproved purity.

To this end, the invention provides a method of producing metal or metalalloy granules, comprising a step consisting in:

-   -   preparing a metal or a metal alloy having non-metallic        inclusions essentially comprising oxides of the base metal;    -   pelletizing the metal or the alloy with a reducing agent in        order to form the granules;    -   processing the granules in a vacuum so that the reducing agent        reacts on the inclusions; and    -   eliminating a surface layer from the granules.

The Applicant has discovered, surprisingly, that eliminating the surfacelayer increases the overall purity of the granules. This result appearsto be paradoxical since surface elimination cannot have any effect onthe core composition of the granules. Nevertheless, it can be explainedby the fact that the concentration of impurities is greater at theperiphery of a granule than in its center. Surface elimination thusremoves a greater fraction of impurities. It thus turns out that thisstep of elimination, in particular by tribofinishing, is technicallyhighly advantageous. In addition, it enables the appearance of theproduct to be improved.

The method of the invention may also present at least one of thefollowing characteristics:

-   -   elimination comprises abrasion;    -   the method includes tribofinishing;    -   elimination is performed by means of a vibrating enclosure;    -   the thickness of the eliminated layer lies in the range 0.1        millimeters (mm) to 0.5 mm;    -   the metal is selected from chromium, titanium, vanadium,        molybdenum, manganese, niobium, tungsten, and nickel, and the        alloy comprises at least one of the above metals and/or boron;    -   the alloy is a ferro-alloy;    -   the preparation step makes use of an aluminothermic reaction        between at least one metal oxide and divided aluminum;    -   the reaction is unbalanced due to a shortage of aluminum        relative to the quantity of aluminum needed for a complete        reaction so as to ensure that the metal or the alloy contains        reducible non-metallic inclusions mainly constituted by        inclusions of the oxide of the base metal;    -   after pelletizing, the granules are baked, in particular at a        temperature lying in the range 200° C. to 230° C.;    -   the reducing treatment is performed in a vacuum oven; and    -   after the reducing treatment, the product is cooled in a neutral        atmosphere.

Other characteristics and advantages of the invention appear furtherfrom the following description of a preferred implementation.

In this example, the purpose is to produce metallic chromium of highpurity by an unbalanced aluminothermic reaction. It mainly implementsthe following four steps a), b), c), and d).

Step a)

Chromium oxide (Cr₂O₃), potassium bichromate (K₂Cr₂O₇), and dividedaluminum are introduced into an aluminothermic crucible lined with arefractory material. The chromium oxide is advantageously acommercially-available product having grain size lying in the range 0 to15 micrometers (μm), while the divided aluminum and the potassiumbichromate are constituted by grains smaller than 1 mm.

The chromium oxide and the potassium bichromate are present instochiometric proportions for the aluminothermic reaction, whereas thealuminum is present with a shortage relative to the stochiometricproportion for the conventional aluminothermic reaction. This shortageof aluminum may lie in the range 0.5% to 8% by weight, and preferably inthe range 2% to 5% by weight, of the stochiometric quantity.

These three ingredients are weighed out and mixed together carefully andthen the reaction is initiated in the crucible in appropriate manner.The reaction temperature quickly reaches a value of about. 2200° C., andat the end of the reaction, metal is recovered from the bottom of thecrucible, with slag floating on top.

Step b)

The metallic chromium obtained in step a) is ground in an impactgrinder, advantageously constituted by a hammer-type high energy grinder(moving hammers striking stationary hammers) until a fine powder isobtained that can pass entirely through a screen having a mesh size of500 μm, and that does not pass through a screen having a mesh size of 77μm.

In the present example, the grinding is purifying grinding that producesventilation, i.e. sweeping by a certain flow of air. The air flow ratecan be adjusted voluntarily in order to accentuate the purifying effect,if so desired. Similarly, this purifying effect can be associated withusing screening or any other kind of selective separation to eliminatethe finest particles in ground material, since the vast majority of thenon-metallic inclusions released by the grinding are to be foundconcentrated in the finest particles.

The resulting purified chromium powder is then intimately mixed with areducing agent and a pelletizing agent. The pelletizing agent isadvantageously constituted by a mixture of Bakelite and an organicbinder such as furfuraldehyde. The purpose of this agent is tofacilitate making pellets at low temperature, with the Bakelitedissolved in the binder forming a cold adhesive, and also serving tofacilitate subsequent polymerization of the Bakelite at highertemperature. Naturally, it is possible to use other thermosettingpelletizing agents and other solvents. The reducing agent isadvantageously constituted by carbon black, and serves to add to thecarbon in the Bakelite.

The respective quantities of these ingredients can be varied, butoverall they are adjusted, with a small amount of excess, to match theresidual oxygen content of the grounds. By way of example, the mixtureof reducing agent and pelletizing agent may be constituted by 0.1%Bakelite, 0.3% furfuraldehyde, and 0.05% to 0.2% carbon black, thesepercentages being relative to the weight of the grounds.

The resulting mixture is shaped into pellets or tablets by means of aconventional compacting press, such as a pelletizing press withtangential wheels or a tablet press. After being pelletized, the mixtureis baked at a suitable temperature (about 200° C. to 230° C.) in orderto eliminate the organic binder and polymerize the Bakelite which formsa binder and imparts strength to the pellets or tablets.

Nevertheless, it should be observed that the baking temperature shouldbe restricted to the minimum required so as to avoid any oxidation ofthe product.

Step c

The pellets or tablets obtained in the preceding step are then subjectedto reducing treatment at 1100° C. to 1400° C. under a high vacuum ofabout 133×10⁻⁴ pascals (Pa).

At the beginning of the vacuum heating cycle, the Bakelite decomposes ata certain temperature leaving a carbon skeleton that is added to thecarbon black that was introduced into the mixture as a reducing agent.On reaching the treatment temperature, this carbon reacts on the oxygenin the Cr₂O₃ that remains in the material, but reacts hardly at all onthe oxygen of the alumina Al₂O₃.

The vacuum in the treatment furnace is then taken to 133×10⁻¹ Pa bycontrolled sweeping of a non-oxidizing gas or a reducing gas, such ashydrogen, which has the particular feature of being practicallyinsoluble in solid chromium.

Because of the relatively low vacuums and the relatively lowtemperatures imposed by chromium subliming, the treatment can takeseveral hours before achieving almost complete reaction.

Step d)

During this subsequent step, a surface layer is eliminated from thepellets by light abrasion. This step is implemented specifically bytribofinishing. To do this, a tribofinishing machine is used that isconstituted by a lined bowl which is set into vibration by means ofunbalanced motors. Under the effect of the vibration, the pellets erodeagainst one another.

Tribofinishing lasts for about 10 minutes. The thickness of the abradedlayer lies in the range 0.01 mm to 0.5 mm.

Not only does this step contribute to improving the appearance of thepellets by polishing them, but it also eliminates a large quantity ofthe impurities.

The eliminated impurities are mainly O₂ and N₂.

The Applicant has performed experiments on 30 pairs of chromium pelletsamples. The Applicant has measured the general content of atoms ofoxygen, nitrogen, and carbon in one sample in each pair that was notsubjected to tribofinishing. The same measurements were performed on theother sample of each pair after being subjected to tribofinishing. Theresults are summarized in the following table. O N C Before 391 36 184(1) tribofinishing After 360 30 180 (2) tribofinishing Difference 31 6 4(3)

In this table, “O”, “N”, and “C” represent atoms of oxygen, nitrogen,and carbon.

The values given represent the quantities of the components in parts permillion.

Lines (1) and (2) give mean values taken over samples respectively withand without tribofinishing. Line (3) gives the difference between lines(1) and (2).

It can be seen that the mean improvement is 31 ppm for oxygen (i.e. 8%)and 6 ppm for nitrogen (i.e. 16%). It is not so good for carbon.

The tribofinishing step thus enables the purity of the pellets to beimproved.

Naturally, the invention is not limited to the preferred implementationdescribed above.

Thus, step a) can be performed other than by aluminothermically, e.g.silicothermically, or by reduction in an electric furnace, in order toobtain a metal or an alloy including non-metallic inclusions in the formof oxides of the base metal.

For a silicothermic reaction, mention can be made, as non-limitingexamples, of producing ferro-chromium or metallic chromium by reductionwith metallic silicon or silicon-chromium, and also of preparingferro-tungsten or ferro-molybdenum by reduction with a high-contentferro-silicon or with metallic silicon.

For reduction in an electric furnace, mention can be made by way ofnon-limiting example, of producing ferro-vanadium in an electricfurnace, followed by an aluminothermic reaction.

The elimination step may be implemented by means other thantribofinishing, for example by polishing, grinding with emery,shot-peening, or sand blasting.

Similarly, step a) can be performed other than by an aluminothermicreaction, for example by a silicothermic reaction, or by reduction in anelectric oven, in order to obtain a metal or an alloy includingnon-metallic inclusions in the form of oxides of the base metal.

1. A method of producing metal or metal alloy granules, characterized inthat it comprises the steps consisting in: preparing a metal or a metalalloy having non-metallic inclusions essentially comprising oxides ofthe base metal; pelletizing the metal or the alloy with a reducing agentin order to form the granules; processing the granules in a vacuum sothat the reducing agent reacts on the inclusions; and eliminating asurface layer from the granules.
 2. A method according to claim 1,characterized in that elimination comprises abrasion.
 3. A methodaccording to claim 1 or claim 2, characterized in that it includestribofinishing.
 4. A method according to any one of claims 1 to 3,characterized in that elimination is performed by means of a vibratingenclosure.
 5. A method according to any one of claims 1 to 4,characterized in that the thickness of the eliminated layer lies in therange 0.1 mm to 0.5 mm.
 6. A method according to any one of claims 1 to5, characterized in that the metal is selected from chromium, titanium,vanadium, molybdenum, manganese, niobium, tungsten, and nickel, and thealloy comprises at least one of the above metals and/or boron.
 7. Amethod according to any one of claims 1 to 5, characterized in that thealloy is a ferro-alloy.
 8. A method according to any one of claims 1 to7, characterized in that the preparation step makes use of analuminothermic reaction between at least one metal oxide and dividedaluminum.
 9. A method according to claim 8, characterized in that thereaction is unbalanced due to a shortage of aluminum relative to thequantity of aluminum needed for a complete reaction so as to ensure thatthe metal or the alloy contains reducible non-metallic inclusions mainlyconstituted by inclusions of the oxide of the base metal.
 10. A methodaccording to any one of claims 1 to 9, characterized in that afterpelletizing, the granules are baked, in particular at a temperaturelying in the range 200° C. to 230° C.
 11. A method according to any oneof claims 1 to 10, characterized in that the reducing treatment isperformed in a vacuum oven.
 12. A method according to any one of claims1 to 11, characterized in that after the reducing treatment, the productis cooled in a neutral atmosphere.